Enteropathy-associated T cell lymphoma (EATCL; malignant histiocytosis of the intestine) arises in patients with enteropathy, which in some cases is known to be a result of gluten sensitivity. The lymphoma arises in the intestine, where it may remain localized, although eventual dissemination is the rule. Intraepithelial tumour cells often are seen at the mucosal tumor margin. These features suggest that EATCL may be a tumor of intraepithelial lymphocytes. A monoclonal antibody (HML-1) has been produced recently that recognizes the entire intraepithelial lymphocyte population and 50% of lamina propria T cells but very few cells outside the mucosa. Immunocytochemistry has shown that all cases of enteropathy-associated T cell lymphoma studied are HML-1 positive and all peripheral T cell lymphomas and mucosal B cell lymphomas are HML-1 negative. This suggests strongly that EATCL is a tumor of mucosal T cells, possibly the intraepithelial T cell component.